The invention relates to an electronic flash of series controlled type, and more particularly, to such an electronic flash which includes a switching element connected in series with a circuit loop to feed a flash discharge tube from a main capacitor and which is turned on and off to control the emission of flashlight from the tube.
A conventional electronic flash of series controlled type is constructed in a manner as illustrated in FIG. 1. Specifically, a booster power supply 10 comprising a DC/DC converter which converts the electromotive force of a low voltage source such as a dry cell to a higher value has its negative terminal connected to a bus l.sub.1 and its positive terminal connected to a bus l.sub.2 through a rectifier diode D1. A main capacitor C.sub.M is connected across the buses as is a charging complete indicator circuit formed by a series combination of a resistor R1 and a neon tube N. A starter circuit 20 is also connected across the buses l.sub.1, l.sub.2, and a series combination of a flash discharge tube XL and a main thyristor SCR1 is also connected thereacross. The junction between the cathode of the discharge tube XL and the anode of the thyristor SCR1 is connected through a series combination of a commutating capacitor Cc and a commutating thyristor SCR2 to the bus l.sub.1. The opposite ends of the capacitor Cc are connected to the buses l.sub.1, l.sub.2, respectively, through resistors R2, R3, respectively, thus allowing this capacitor to be charged. The commutating thyristor SCR2 has its gate connected to a control output of a photometric circuit 30.
Synchro contacts X which are disposed within a camera, not shown, have their one terminal connected to an input of the starter circuit 20. The starter circuit 20 has a first control output which is connected to the trigger electrode XL.sub.T of the flash discharge tube XL and a second control output which is connected to the gate of the main thyristor SCR1.
In operation, when the synchro contacts X are closed, a high voltage trigger signal a is applied to the trigger electrode XL.sub.T of the discharge tube XL from the first control output, and simultaneously a proper control signal b which is effective to fire the main thyristor SCR1 is applied from the second control output of the starter circuit 20. The flash discharge tube XL then initiates the emission of flashlight. An object being photographed is illuminated by such flashlight, and reflected light therefrom impinges upon the photometric circuit 30 which is then operative to provide an integral of such reflection until the integral reaches a value which is sufficient to provide a proper exposure, whereupon the photometric circuit 30 delivers, at its control output, an emission terminate signal c which is effective to fire the commutating thyristor SCR2. Accordingly, as the thyristor SCR2 is fired, the commutating capacitor Cc which has been charged through the resistors R2 and R3 now reversely biases the main thyristor SCR1, thus turning it off to terminate the emission of flashlight from the dishcarge tube XL.
In the described operation of the conventional electronic flash, the luminance of emission from the discharge tube XL may be represented as shown in FIG. 2.
Referring to FIG. 2, the emission is initiated at time t.sub.0, and the terminate signal c is generated at time t.sub.1. After time t.sub.1, or after the commutating thyristor SCR2 is fired to turn the main thyristor SCR1 off, the commutating capacitor Cc begins to be charged to the opposite polarity through a path including the bus l.sub.2, the discharge tube XL, commutating capacitor Cc, commutating thyristor SCR2 and the bus l.sub.1. This means that an excess amount of emission occurs as indicated by a hatched area in FIG. 2. The greater the amount of charge which remains on the main capacitor C.sub.M, or the less the distance to an object being photographed, the degree of excess emission will be greater. Hence, if a camera has a preset F-value of "4", the optimum F-value will increase toward the smaller distance, as indicated in FIG. 3 graphically, and hence there will result an overexposure if the preset F-value is used.
While there is proposed the provision of a coil between the bus l.sub.2 and the discharge tube XL so as to smooth out the rising and falling response of the emission luminance to thereby reduce the excess emission, the described problem cannot be solved even though it is effective to a degree.
To accommodate for this, there is a proposal (see Japanese Patent Publication No. 17,333/1973), as shown in FIG. 4, in which a thyristor SCR3 is connected between the bus l.sub.2 and the junction between the commutating capacitor Cc and resistor R3, and in which a diode D2 is connected between the junction between the commutating capacitor Cc and resistor R3 and the junction between the discharge tube XL and the main thyristor SCR1. This arrangement permits the thyristor SCR3 to be fired in response to a signal d from the photometric circuit 30 at the same time as the main thyristor SCR1 is turned off, allowing the majority of the reverse charging current to the commutating capacitor Cc, which occurs after the main thyristor SCR1 is turned off, to pass through the thyristor SCR3, thus minimizing the current flow through the discharge tube XL. In this manner, the excess emission is reduced as compared with the circuit arrangement shown in FIG. 1. However, because the thyristor SCR3 is connected in parallel with the discharge tube XL, the magnitude of the excess emission is not reduced to a negligible value, and hence the problem remains unsolved.